It is now widely known to provide one or more inflatable airbags within the interior compartment of a motor vehicle, for inflation upon receipt of an appropriate signal from a crash sensor indicative of an actual or predicted accident, in order to provide protection to the driver or other occupants of the motor vehicle. Typically, airbags of this nature are constructed from woven fabric, and are initially folded and/or rolled into a tight package provided behind a rupturable cover in an airbag module. The folded airbag is fluidly connected to an inflator such as a gas generator which is configured to generate a large volume of inflating gas and to direct that gas into the airbag upon receipt of said crash-signal.
As will be appreciated, given the very short period of time within which an airbag must inflate in order to provide adequate protection to a vehicle occupant in the event of a crash, the flow of gas used to inflate the airbag can be very powerful, and is also generally very hot. It is therefore conventional to provide the fabric of the airbag with a surface coating in order to impart flame resistant properties to the fabric, and also to manage the permeability of the fabric to the inflating gas and particles produced by the gas generator. In this regard, it is known to use woven fabric of polyamide material, and to coat the fabric with a silicone coating which has been found to have good levels of impermeability to gases and particles, and to have relatively good long term aging performance. However, it has been found that there are a number of problems with conventional woven airbag fabrics when coated in this way.
For example, conventional woven fabrics have a surface which is slightly uneven, as a consequence of the difference in height between the peaks and troughs of the yarns as the yarns are woven around each other. This can be particularly pronounced in so called “one-piece-woven” airbags comprising two layers of fabric, the two layers being combined in selected areas as they are woven, to form an integral seam in which yarns forming one of said layers are interwoven with yarns forming the other said layer.
An exemplary method of weaving a one-piece-woven airbag is disclosed in EP0458838A.
Another problem that arises with conventional one-piece-woven fabrics used for airbags is that they have a significant inherent angle between yarns where the yarns are interlaced, the angle being caused by wrapping the yarns around one another. This angle causes the cross-sectional thickness of the fabric to increase, and also causes localized gaps to form between yarns. This necessitates a thicker layer of coating with a resulting increase in the weight of the coating in order to achieve the desired permeability, again increasing the weight and cost of the fabric, and also resulting in a fabric which is less susceptible to very tight folding for packaging purposes.
As will be appreciated, it is generally favorable for airbag fabrics to be as lightweight and flexible as possible, so that they can be folded very tightly into small airbag modules, and so that they can be inflated by relatively low volumes of gas, thereby requiring smaller inflators. However, this desire for a lightweight woven fabric must be balanced with the requirement for the fabric to have a sufficient inherent impermeability to gas, or else the fabric will require a thicker coating, thereby negating the effect of its light weight.
While lightweight fabrics can easily be woven with a relatively open weave (i.e. low sett), thereby reducing the mass of yarn material per unit area, the resulting open weave is less susceptible to coating, for the reason that the coating must seal larger interstices between adjacent yarns.
It is therefore an object of the present invention to provide an improved airbag. It is another object of the present invention to provide an improved method of making an airbag.